Regenerative fuel cells for energy storage are well known, commonly using hydrogen and oxygen as the reacting gases. These cells have several disadvantages, the principal one being that the high overvoltage for the oxygen reaction in either direction, charge or discharge, limits the energy storage efficiency to about 55 to 60% compared to energy storage efficiency of secondary batteries of about 70%. The high overvoltage also causes severe corrosion of catalysts and support materials limiting the life of the cathode. This corrosion may well require separate fuel cells for charge and discharge.